Fragmenting tube energy absorber

ABSTRACT

A frangible tube energy absorber is described including means for inhibiting propagation of longitudinal fracture of the tube. In a preferred embodiment the means for inhibiting propagation of longitudinal fracture comprises means for applying a radially inwardly directed force against the outside of the tube also inducing a higher frequency of transverse cracking to increase the amount of energy absorbed as well as improve the reliability and reproducibility.

United States Patent Donald H. Wykes Downey, Calif.

Oct. 3, 1968 Sept. 28, 1971 North American Rockwell Corporation El Segundo, Calif.

inventor Appl. No. Filed Patented Assignee FRAGMENTING TUBE ENERGY ABSORBER 15 Claims. 14 Drawing Figs.

U.S. Cl 188/1, 244/100 Int. Cl F161 7/l2 Field of Search I88/ 1 C; 244/100 References Cited UNITED STATES PATENTS 3,143,321 8/1964 McGehee et al 188/1 C X 3,236,333 2/1966 Mitchell l88/l C Primary Examiner-Duane A. Reger Attorneys-William R. Lane, Allan Rothenberg and Richard D. Seibel mama) sins m1 SHEEI 2 [IF 3 FIG.8C

FIG. 8B

INVIZN'IOR.

DONALD H. WYKES him! 5. (Li? FIG. 80

AT TORNE Y mama) was um SHEET 3 OF 3 Fla. 9

FIG. IO

INVINFOR FIG. I DONALD H. WYKES flfzm Zap AT TORNE Y FRAGMENTING TUBE ENERGY ABSORBER BACKGROUND Frangible tube energy dissipation devices have been found advantageous in many applications because they are highly efficient in tenus of the quantity of energy dissipated per unit such as aluminum alloy, steel or plastic reinforced with glass fiber, boron fibers, graphite or the like, is forced onto a die or anvil having an outwardly flaring radius acting against the interior of the tube upon stroking thereof to enlarge the tube beyond its ultimate strength thereby causing fracturing of the tube. The die radius rapidly and nonlinearly flares outwardly so that a biaxial stress is applied to the tube thereby causing both longitudinal and transverse cracking and hence fracture into a plurality of fragments. In this process energy is absorbed both in the plastic deformation and in fracturing of the tube. The quantity of energy absorbed is determined by the strength of he tube material, the crack propagation properties of the material, and the geometry of the die.

Another type of energy absorber employs a tube that is plasticly deformable and a die that splits the tube longitudinally with the ribbons so formed being curled. Longitudinal slits may be provided in such tubes to promote longitudinal splitting. The energy absorbed is a function of the yield strength of the tube material, whereas, in the frangible tube absorber, it is also a function of the ultimate strength of the, tube material. Since the frangible tube is only plasticly deformed but also fractured transversely, greater energy absorption per unit weight may be achieved. In addition to employing different tube materials for fragmenting tube absorbers as compared with deformable tube absorbers, the dies are of somewhat different design to effect the two diverse types of energy absorption.

In the past the fragmenting tube type of energy absorber has not reached its full efficiency in terms of energy absorbed per unit of weight because control over the mode of fragmentation has been poor. This lack of control results in erratic load fluctuations during tube deformation and rather poor tube-to-tube reproducibility. Occasionally a crack will propagate substantially the full length of the tube or plastic deformation will take placed during stroking resulting in a minimal amount of fracturing and a relatively low value of energy absorption. These difficulties are particularly pronounced in high-strength tubes (for example, 250,000 p.s.i. ultimate) and in the tube-wall thickness to die radius ratios which should give the maximum energy absorption.

SUMMARY OF THE INVENTION Thus in the practice of this invention according to a preferred embodiment there is provided a frangible tube energy absorber comprising a frangible tube, an enlarging die at one end of the tube for fracturing the tube into a plurality of fragments, and means for controlling the mode of fracturing the tube.

DRAWINGS Attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates, in perspective, a frangible tube energy absorber incorporating principles of this invention with a stretchable torus surrounding the tube for inhibiting propagation of longitudinal fracture;

FIG. 2 shows a longitudinal section of the tube of FIG. 1;

FIG. 3 comprises a cross-sectional view of an energy absorber having rollers for inhibiting propagation of longitudinal fracture;

FIG. 4 comprises a top view of the energy absorber of FIG.

FIG. 5 illustrates a roller with ridges for use in the embodiment of FIG. 3;

FIG. 6 illustrates an alternative ridged roller for use in the embodiment of FIG. 3;

FIG. 7 shows a frangible tube energy absorber having a pattern of shallow grooves for controlling fracture;

FIGS. 8A to 8D illustrates alternative patterns of grooves for the frangible tube of FIG. 7;

FIG. 9 shows a cross section of a frangible tube energy absorber having a fixed ring for restraining an end of the tube;

FIG. 10 comprises a top view of the energy absorber of FIG. 9; and

FIG. 11 comprises a cross section view of a fixed ringrestraining member with a plurality of balls in the ring for engagement with thetube upon stroking.

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION FIG. 1 illustrates a frangible tube energy absorber incorporating the principles of this invention. As illustrated in this embodiment there is provided a die or anvil 10 onto which a frangible tube 11 is pressed during operation of the absorber in substantially the same manner described and illustrated in the aforementioned US. Pat. No. 3,143,321. Upon stroking of the tube onto the die 10 the tube is fractured into a plurality of fragments 12 thereby absorbing energy in the bending and rending of the tube. This operation can also be seen in FIG. 2 wherein the tube is moved downwardly onto a die radius 13 on the die 10 (or the die moved upwardly onto the tube). The die radius 13 nonlinearly flares the end of the tube 11 as the tube is stroked downwardly thereby imparting large magnitude biaxial stresses on the tube yielding large plastic flow and many fractures 14 extending both tranversely and longitudinally of the tube to produce a plurality of separate discontinuous fragments 12.

The embodiment of FIGS. 1 and 2 also includes a torus 16 of elastically stretchable material, such as high-strength rubber, or other elastomer, surrounding the tube at a point directly opposite the die radius 13 during operation of the energy absorber or dissipater. If desired the rubber can be reinforced with glass fibers or the like to increase the elastic modulus. The original position of the frangible tube and torus relative to the die before stroking of the tube is shown in phantom in FIG. 2. Thus it can be seen that the torus is originally arranged near the end of the tube near a tapered end 15 (such a tapered end is conventionally employed to permit gradual increases of force rather than an instantaneous increase of force as would be obtained with a blunt-ended tube).

Upon stroking of the tube towards the die the torus is carried along with the tube down to a point opposite the die radius due to frictional engagement with the tube. When the torus reaches the bottom of its possible travel a substantial portion of the surface is frictionally engaged with the tube as the tube flares outwardly. This causes a rolling of the torus along the length of the tube as the tube progresses through the stretchable torus. This action is substantially identical to that commonly obtained in rolling' a rubber O-ring onto an unlubricated shaft. The torus has an internal surface portion moving with the tube opposite the die radius and this portion then moves outwardly opposite the outward flare of the die, then moves upwardly in the direction opposite to the direction of tube-stroking and finally moves back inwardly to reengage the tube.

In rolling on the tube opposite the die radius an inwardly and downwardly directed force is applied to the tube by the torus. This force constraining outward expansion of the tube at a critical region at the commencement of the die radius acts to reinforce the tube in a region subjected to high stresses tending to induce longitudinal cracking. The presence of the restraining force directed radially inwardly on the tube by the torus inhibits the propagation of longitudinal fractures above the die radius thereby substantially reducing premature tube failure for enhancing the reproducibility and efficiency of energy absorption from one tube to another. Inhibiting longitudinal cracking in the tube also encourage transverse cracking with enhanced energy absorption as compared with an unrestrained tube. Pressure of the elastically deforming torus inwardly and downwardly into the die radius forces the material of the tube to fragment more frequently into smaller fragments of more consistent size and geometry in both the longitudinal and transverse directions and restricts longitudinal fragmentation of the tube to a region well into the die radius, thereby effectively restraining the propagation of cracks that yield erratic fragmentation and thereby preventing large fluctuations in load during tube stroking. Rolling of the torus along the tube provides a continuing inward force on the tube without a large increase in friction on the tube so that a substantial tube length can be used without increasing the possibility of tube buckling.

FIG. 3 illustrates in cross section an alternative embodiment of restraining means for the tube in a frangible tube energy absorber wherein the restraining means, as with the torus, includes a portion that is temporarily movable longitudinally with the tube opposite the die radius upon stroking of the tube. Thus in the embodiment of FIGS. 1 and 2 the portion of the torus opposite the die radius is temporarily moving with the tube and subsequently moves out of contact with the tube and around the outside of the toroidal ring to again engage the tube and move therewith near its entry into the die radius. In the embodiment of FIGS. 3 and 4 a similar effect is obtained by providing a plurality of rollers 17 surrounding the tube upon stroking and acting against its outside surface at a location opposite the die radius to effectively restrain the tube from expanding radially until well into the die radius thereby inhibiting propagation of longitudinal cracking of the tube beyond the die and increasing the frequency of failures to provide a more uniform stroking force and energy absorption during the length of the stroke. By increasing the frequency of failures smaller fragments are obtained and variations between one fracture and the next are smoothed out so that a more uniform load-versus-displacement or force-versusstroke relation is obtained.

As is more clearly seen in the top view of FIG. 4, each of the rollers 17 is journaled at each end in a roller support 18 so as to be maintained in contact with the outside of the tube. The roller supports 18 are radially extending ribs in which a bearing extension 19 on the roller is journaled. If desired, the roller supports need not extend outwardly to the maximum diameter of the anvil. The roller supports 18 are bolted to the die or anvil along with a debris cover 20 which serves to prevent fragments of the tube from jamming between the tube and rollers.

of the rollers 17 has a concave" face bearing on the tube; that is, the diameter of the roller is larger at the ends than in the middle so that the face of the roller mates with the outside diameter of the tube. By selecting a die radius appropriate to the rollers, the rollers are maintained in contact with the tube during a substantial portion of the flaring. It will be apparent that in operation the rotation of the roller due to motion of the tube cannot be with the same peripheral speed at all points and hence some portions of the roller face must be slipping relative to the tube. This slipping and the consequent nonuniform frictional engagement between the rollers and tube may cause local deformations which enhance the energy absorption and further enhance the favorable fracture patterns in the tube.

In order to take full advantage of the nonuniform frictional engagement between the roller and the tube and promote more controlled fracturing it is desirable in some circumstances to employ a roller having a plurality of raised portions to selectively engage the tube with higher forces in some regions than in others. A roller so formed is illustrated in FIG. 5. As illustrated therein, the roller 21 includes a plurality of raised triangular ridges 22 extending along the face of the roller in a longitudinal direction. It will be apparent that the longitudinal ridges 22 on the roller are transverse to the longitudinal axis of the tube when employed in an embodiment as illustrated in FIG. 3.- In operation, a roller as illustrated in FIG. 5 operates against the outside surfaces of the tube with the raised ridges serving as stress concentration points about which bending may preferentially occur thereby inducing fracture at corresponding locations in the tube. The transverse cracks so initiated induce very uniform fracturing of the tube upon stroking thereof for optimum uniformity of energy absorption. The transverse cracks also serve to interrupt longitudinal cracks in the tube thereby effectively preventing propagation of cracks along the full length of the tube to cause failure at low stress.

The rollers are journaled so as to fit tightly against the tube upon stroking, and if desired spring loading can be employed. The rollers may be hardened steel and sufficiently tight against the tube to indent the surface. This is not necessary and good results are obtained with relatively rigid rollers of hard rubber or plastic. The action of the roller is to afford a stress-raising point in the manner of a fulcrum and indenting is not necessary for this in every instance.

An alternative arrangement of roller with an irregular face in the form of raised ridge is illustrated in the embodiment of FIG. 6. As illustrated in this embodiment a plurality of short raised ridges 24 are provided in rows on the face of a roller 25 for engaging the outside surfaces of a tube as hereinabove described in relation to FIG. 3. The raised ridges 24 which are also triangular in cross section, are arranged in rows with each of the ridges being of relatively short length compared with the length of the roller and each of the raised ridges in one row is staggered from the ridges in the adjacent rows. It will be readily appreciated by one skilled in the art that other patterns of raised ridges may be provided on roller surfaces to act as 20 sources to assure uniform fragmentation.

In order to control fracture patterns of the tube indentations in the surface may be provided throughout the length of the tube or in a selected portion thereof rather than relying on raised ridges on a roller or a combination of both tube indention and roller ridges may be employed. A tube so formed is illustrated in FIG. 7 wherein a tube 27 has shallow grooves 28 in the external surface for increasing stress and controlling fracture patterns. In the tube 27 the grooves are arranged in circumferential lines around the tube and in short longitudinal segments along the length of the tube between the circumferential grooves to yield an overall pattern similar to that commonly employed in laying brick. A pattern of this nature promotes both longitudinal and transverse fracturing without providing a direct line for longitudinal cracking of the tube since the longitudinal grooves are not extensively continuous and extend only a short length along the tube. Uniform and reproducible fragmentation is obtained by such texturing of the tube surface.

It will be apparent that other patterns can be employed on the tube surface as illustrated in FIG. 8. Thus, as shown in FIG. 8A, a pattern of uniform squares may be employed on the tube surface. Similarly, as shown in FIG. SE, a series of short grooves transverse to the tube axis may be formed on the surface in a pattern similar to that formed by the roller hereinabove described and illustrated in FIG. 6. Still another pattern of surface texturing is illustrated in FIG. 8C wherein a plurality of short grooves is provided transverse to the tube axis. In this embodiment the grooves are arrayed in rows with alternate rows of grooves having ends overlapping the ends of grooves in the intermediate rows to inhibit longitudinal crack propagation Patterns such as those illustrated are readily machined, embossed, or chemically etched into the tube surface on either the external or internal portion. It is also contemplated to provide a pattern of grooves on the internal surface of the tube since crack initiation during tube fracturing is initiated at the inner surface. A pattern of grooves particularly suitable for forming on the inside of the tube is illustrated in FIG. 8D wherein multiple long pitch, rightand left-hand threads are overlapped to produce a diamondlike pattern on the surface. Such a pattern of fracturing affords high-energy absorption since the relative crack length for each fragment is greater than the crack length for substantially square fragment of the same volume.

It will be apparent that many other patterns can be provided on either the inside or outside of the tube for specific situations and that various patterns and fragment-promoting sizes of pattern could be combined sequentially along the length of a tube to change the load/deformation curve along the length to suit specific energy-absorbing requirements.

FIG. 9 illustrates in cross section a frangible tube energy absorber having a fixed ring restraining an end of the tube adjacent the die radius for inhibiting propagation of longitudinal cracks in the tube. As illustrated in this embodiment, an anvil or die 31 is employed for spreading and fracturing a tube 32. A ring 33 is rigidly secured to the die 31 by a plurality of bolts 34 and is spaced from the anvil at a fixed distance by a plurality of spacers 36 which are hidden from view in the top view of FIG. 10.

As the tube 32 is pressed onto the die 31 it is constrained in the die radius 37 by the ring 33 which acts in substantially the same manner as the rollers herein described and illustrated in FIGS. 3 and 4. It will be apparent that the ring does not have a portion moving with the surface of the tube during stroking thereof into the die and that a substantial friction force may be involved between the ring and tube.

In order to minimize the friction force between a fixed ring and the tube a low-friction coating such as Teflon (polytetrafluoroethylene or fluorinated ethylene propylene) may be applied to either the tube or ring to lubricate the interface therebetween. The friction between the ring and tube in the die radius can be alleviated by providing a plurality of captured balls within the ring as illustrated in the cross-sectional view of FIG. 11. As illustrated therein a tube 39 is pressed onto a die 40. A ring 41 surrounds the tube at the die radius for inhibiting longitudinal cracking and promoting transverse cracking as hereinabove described. Within internal cavities of the ring 41 a plurality of freely rotatable balls 42 are provided which bear on the tube opposite the die radius, upon stroking of the tube. The balls are preferably closely spaced to provide support for the tube substantially completely around its periphery and if desired, the balls on the tube may be coated with a thin layer of Teflon or other suitable solid lubricant for minimizing friction.

Obviously many variations may be made in the structure described within the principles of this invention. Thus, for example, the frangible tube can readily be combined with other types of energy absorbers in a most efficient manner. A core of crushable honeycomb or balsa wood can be provided within the tube and the crushing thereof by the member stroking the tube enhances energy absorption without substantial increase in weight. Likewise a second frangible tube can be arranged concentrically with the first to increase energy absorption without substantial increase in weight. The frangible tube arrangement provided in practice of this invention can include a plurality of rolling rings and an outer constraining cylinder in the manner of the energy absorber developed by Mechanics Research, Inc., wherein the rolling rings are plastically deformed between the containing walls for energy absorption.

What is claimed is:

1. A frangible tube energy absorber comprising:

a frangible tube;

a die for one of said tube and including a die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die;

means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally with said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and

comprising an elastically stretchable torus around said tube and having an internal surface portion engaging said tube.

2. An absorber as defined in claim 1 wherein said torus comprises an elastomeric ring having sufficient elastic elongation to roll along said tube.

3. A frangible tube energy absorber comprising:

a frangible tube;

a die at one end of said tube and including a die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die;

means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and

comprising a plurality of rollers around said tube each of said rollers having a concave tube engaging face with a constricted center portion and enlarged ends.

4. An absorber as defined in claim 3 wherein said rollers each comprise a plurality of raised portions on said concave face for controlling fracture patterns of said tube.

5. An absorber as defined in claim 4 wherein said raised portions include ridges arranged transverse to the axis of said tube for inhibiting propagation of longitudinal fractures of said tube above said die.

6. An absorber as defined in claim 5 wherein said ridges are continuous substantiallyentirely along the length of the roller face.

7. An absorber as defined in claim 5 wherein said ridges are intermittent along the length of the roller face and are arranged in rows with raised portions in successive rows being staggered from raised portions in preceding rows.

8. A frangible tube energy absorber comprising:

a frangible tube;

a die at one end of said tube and including die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die;

means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally with said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and

said means for applying a force comprising a plurality of balls bearing on said tube.

9. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising:

means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis;

said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius;

said means for applying a force including a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and

a plurality of balls around said tube and bearing on the outside thereof. 10. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising:

means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis;

said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius;

said means for applying a force includes a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and

an elastically stretchable torus in tight engagement around said tube.

11. A frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising:

means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis;

said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius;

said means for applying a force including a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and I a plurality of rollers around said tube and bearing on the outside thereof.

12. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube including a die portion for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising:

indentations fonned on the surface of said tube and disposed at an angle to the tube axis for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis; and

said indentations are stress-raising indentations in a tube surface, said indentations each extending no more than a short length.

13. A frangible tube energy absorber as defined in claim 12 wherein said indentations are formed as said tube is stroked into said die.

14. A frangible tube energy absorber as defined in claim 12 wherein said indentations are formed before said tube is assembled onto said die.

15. A frangible tube energy absorber as defined in claim 14 wherein said indentations are on the outside surface of said tube. 

1. A frangible tube energy absorber comprising: a frangible tube; a die at one end of said tube and including a die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die; means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally with said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and comprising an elastically stretchable torus around said tube and having an internal surface portion engaging said tube.
 2. An absorber as defined in claim 1 wherein said torus comprises an elastomeric ring having sufficient elastic elongation to roll along said tube.
 3. A frangible tube energy absorber comprising: a frangible tube; a die at one end of said tube and including a die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die; means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally with said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and comprising a plurality of rollers around said tube each of said rollers having a concave tube engaging face with a constricted center portion and enlarged ends.
 4. An absorber as defined in claim 3 wherein said rollers each comprise a plurality of raised portions on said concave face for controlling fracture patterns of said tube.
 5. An absorber as defined in claim 4 wherein said raised portions include ridges arranged transverse to the axis of said tube for inhibiting propagation of longitudinal fractures of said tube above said die.
 6. An absorber as defined in claim 5 wherein said ridges are continuous substantially entirely along the length of the roller face.
 7. An absorber as defined in claim 5 wherein said ridges are intermittent along the length of the roller face and are arranged in rows with raised portions in successive rows being staggered from raised portions in preceding rows.
 8. A frangible tube energy absorber comprising: a frangible tube; a die at one end of said tube and including die radius for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward said die; means for applying an inwardly directed force against the outside of said tube adjacent said die radius, said means including a surface portion temporarily movable longitudinally with said tube opposite the die during fracturing of said tube, and wherein said surface portion is temporarily movable longitudinally in a direction opposite to the direction of said tube stroking, whereby propagation of longitudinal cracks above the die portion is inhibited for preventing premature failure of said tube; and said means for applying a force comprising a plurality of balls bearing on said tube.
 9. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising: means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis; said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius; said means for applying a force including a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and a plurality of balls around said tube and bearing on the outside thereof.
 10. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising: means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis; said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius; said means for applying a force includes a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and an elastically stretchable torus in tight engagement around said tube.
 11. A frangible tube energy absorber having a frangible tube and a die at one end of said tube for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising: means for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis; said means for promoting transverse cracks and inhibiting longitudinal cracks comprising means for applying an inwardly directed force against the outside of said tube adjacent said die radius; said means for applying a force including a surface portion temporarily movable longitudinally with said tube during fragmenting thereof; and a plurality of rollers around said tube and bearing on the outside thereof.
 12. In a frangible tube energy absorber having a frangible tube and a die at one end of said tube including a die portion for fracturing said tube into a plurality of discontinuous fragments upon longitudinal stroking of said tube toward such die, the improvement comprising: indentations formed on the surface of said tube and disposed at an angle to the tube axis for both promoting formation of cracks transverse to the tube axis and for inhibiting propagation of cracks parallel to the tube axis; and said indentations are stress-raising indentations in a tube surfacE, said indentations each extending no more than a short length.
 13. A frangible tube energy absorber as defined in claim 12 wherein said indentations are formed as said tube is stroked into said die.
 14. A frangible tube energy absorber as defined in claim 12 wherein said indentations are formed before said tube is assembled onto said die.
 15. A frangible tube energy absorber as defined in claim 14 wherein said indentations are on the outside surface of said tube. 